You can have him have met the other characters in your characters guest stories, that's not really a problem. If the other characters have done something in the campaign where your character might have been a part of, you could retrofit it to have your character have been there, instead of a no-name NPC.
If there is nothing that could fit, discuss this in the group. Maybe there are ways to tie you in with another character between sessions, when he did some offscreen stuff, where the two characters have met. Have your character ready, describe him to the rest of the group and together you should be able to figure out where you might have met.
Alternately, you can leave the guest stories blank and fill them in mid game, when you actually meet the rest of the group. You could come up with aspects before hand, but unless it is linked to a story, the aspect doesn't apply yet. That would give you a sense of what should happen to make a good story for the aspect.